The invention described herein is generally related to angular position sensors that use optical encoding to determine angular position.
Many applications require an angular orientation position sensor which can be read from a distant location. Such applications include industrial process controls and remotely piloted vehicles. Monitoring the angular position of a tool is necessary in many robotic system applications.
One type of remote angle sensor that utilizes polarized optical signals to convey information regarding angular position is described by Migliori, et al in U.S. Pat. No. 4,577,414. In that device, a light beam is split into two inner channel beams and two outer channel beams. The inner channel beams pass through fixed linear polarization filters that are aligned orthogonally to each other. The polarized inner channel beams then pass through a linear polarization filter mounted on a rotatable wheel. The two outer channel beams are directed to fall incident upon concentric semicircular masks printed on the code wheel.
Analog amplitude information is encoded onto the two inner channel beams as the linearly polarized light beams pass through the polarization filter on the code wheel. The only light transmitted through the polarizing filter on the code wheel is linearly polarized at an angle .theta. with respect to the angle of the code wheel polarization filter (.+-.180 degrees). The intensity of the beam transmitted through the polarization filter is proportional to sin.sup.2 .theta. and cos.sup.2 .theta. due to orthogonal orientation of the fixed filters. Letting A equal the intensity of the beam proportional to I.sub.0 sin.sup.2 .theta. and B equal the intensity of the beam proportional to I.sub.0 cos.sup.2 .theta. where I.sub.0 equals the maximum intensity of the beams, the ratio of A to B is equal to tan.sup.2 .theta.. Therefore: EQU .theta.=tan.sup.-1 [(A/B)].sup.1/2 ( 1)
Because both channels are energized by a single light source, the effect of fluctuations in source intensity are eliminated when the ratio A/B is formed. Since light intensity is always a positive quantity, .theta. is always between 0 and 90 degrees. Normalization of the two inner channel intensities is necessary due to differences in the peak intensities of the channels caused by differences in separation between the inner channels, optical path lengths, coupling losses, and electrical component characteristics of these channels. Thus, the Migliori device requires circuitry to store, compare, and update the differences in signal intensities for both inner, analog channels.
Equation 1 assumes both analog inner channels have the same maximum and minimum values. To compensate for the difference in maximum beam intensity between the two analog channels, Migliori modifies Equation 1 so that: EQU .theta.=tan.sup.-1 [{(A.times.B.sub.max)/(B.times.A.sub.max)}.sup.1/2 ](2)
Using the trigonometric identity the angle, .theta., can also be determined from: EQU cos2.theta.=A/A.sub.max -B/B.sub.max
or: EQU .theta.=1/2cos.sup.-1 [A/A.sub.max -B/B.sub.max ] (3)
The two outer channels of the Migliori device provide quadrant ambiguity and are encoded with either "on" or "off" signals depending on the position of the masks on the code wheel relative to the incident outer channel beams. This encoding provides information necessary to determine the position of the code wheel within a quadrant, i.e., 0-90 degrees, 90-180 degrees, 180-270 degrees, or 270-360 degrees.
All four channel beams leave the sensor and are each propagated through separate optical fibers to detector circuitry so that the encoded beams can be processed to provide an output directly in degrees or radians.
Thus, a need exists for a remote angle sensor with a single return channel to bring the encoded signal back to the processing electronics in order to provide a simpler, more compact, and less expensive sensor having greater reliability over present sensors of this type.